first amp build - choice of pre-amp and some really stupid questions

[incurably_optimistic]

hello everyone!
I posted this thread on diystompboxes site because I was more familiar with that messageboard, but I didn't get any responses (because, well, it's a stompbox board, not an amp one) so I figured that this will be a better place to post this stuff:

Hello everyone!

This is my first post so i guess i should introduce myself - I've been making guitar pedals for quite some time but I have never worked with amplifiers, so now is the perfect time to change it I guess! And please excuse me for my poor english as I'm from Poland so I apologize in advance for all my mistakes.

I want to build myself a guitar amplifier. As a first project I would have chosen something small, quiet and simple, but I already have a small 8W amp to play at home, so I thought that maybe I could make myself a practice amp loud enough to play in a band setting? I did some searching and googling and it turned out that a LM3886 chip would be perfect as a power-amp section to get something around 50-60W into 8ohm speaker (but please correct me if it's an overkill or wrong on some other level).

I've had an old & broken solid-state amp laying in the basement, so I disassembled it, threw out the board (after salvaging some parts like 2200uF capacitors or LM1875 chip) and now have myself a nice chassis and neatly wired AC section of the amp. Unfortunately, the transformer included in the amp is not powerful enough for me to use it - it only gives +-17V AC and has a max rating of 45VA, so I'll take it out and use it for some other project.

But back to the topic - now that I've decided on using the LM3886 chip as a power section (I've read that it's loud and clean and that's all that you want from the power section of an amp, right? oh, and it has lots kinds of protections from various failures) I have to choose a preamp to go with it, but I don't really have too much knowledge in this area, so I would like to ask you:

what are the good sounding and simple (as this is my first build it's kind of important) pre-amps to use with a lm3886 power-amp? I was thinking maybe of some low-voltage (or maybe high-voltage?) tube pre-amp - I've never owned a tube-amp so this is kind of a dream of mine, but if there are some great solid-state preamps out there I'm open to suggestions!

I would like it to have a clean and distorted channel, as well as some simple tone control (low, mid and high frequncy controls).

I also have an accutronics spring reverb tank taken from some broken amp that I would also like to throw in there.

For example, I found this project - http://pdfelectronics.com/the-hood-preamp/ and it looks nice for me, but what the hell do i know - what do you think about this one?

Let's say I decided to build the preamp from the link above - can I use one big transformer with secondary voltages 24/0/24 AC to get the +-35VDC needed for LM3886 and then use the +24/0 winding for the pre-amp (as the power supply on the schematic indeed uses a mains to 24V transformer)? Or should I use two separate transformers, one for the power amp and one for the pre-amp?

Sorry for this long post and thanks in advance for all the answers!

after not getting any replies I bumped with own thread with another bunch of stupid questions:

Sorry for bumping my own thread, but I found a schematic for a preamp based on the Fender Twin Reverb amp and it looks really simple and, more importantly, it uses two cheap transformers (one is 12v*0,25A=3W and the second is 12*0,15=1,8W).
Here's the link: http://s198.photobucket.com/user/birdy81260/media/Fender.gif.html
I was thinking about designing the power supply for the whole amp - does it make more sense to use separate power supplies for the preamp (as it uses two small power 230v-12v transformers) and the power amp (bipolar 230Vac primary +-24Vac secondary, power, say, 150VA) or should i use this big transformer to get the +-24Vac needed for the LM3886 and then some other 24V to 230V transformer to crank up the voltage for the tubes? Which solution is better/cheaper/more practical?

I also have a 600ohm accutronics reverb tank taken out from a broken fender amp that I would like to include in this amp, is this preamp powerful enough to drive the tank? If so, then i think i should use some kind of recovery stage, should a single op-amp operating on, say, +-20v be enough for this?

There are a lot of questions in those two posts, so if anyone could answer any of them I would be really thankful!

[phatt]

Nothing wrong with an LM3886 build as a first power amp project,, as long as you use recomended supply setup.
As for preamps,, heck pick one lol. Wise to stay away from glass stuff for now.

A whole lot of great advice and kits here; http://sound.westhost.com/projects.htm
A massive amount of info on those pages and aimed at folks in your situation. hint.

look at **Project 27** a complete 100 Watt Amp build and he sells the kit for same.

Some where there is also a LM3886 kit as well ,,I think,,, his site is so big I can't see it right now but it's there somewhere.

Watch out there is a lot of stuff on the net that Claims to work but some are downright dangerous.
That Hood preamp you linked to is not something I would want to use as it has some potential safety issues.
having built a lot of preamps both Glass and SS I can tell you that valve stuff is not really needed for great sound.
If you want Valves you are better off to go the full build,, All Valves. :tu:

Regards that reverb tank; Yes it's likely useable but more work for you.
Again ESP has one of the best circuits for reverb drivers. look at project 34.
Phil.

[incurably_optimistic]

thanks for your answer!

Nothing wrong with an LM3886 build as a first power amp project,, as long as you use recomended supply setup

Speaking of recommended power supply, when I google "lm3886 power supply" most results use the transformer with two separate secondary windings, while on the page you linked there is a center tapped transformer http://sound.westhost.com/project04.htm - which one is more preferable here? What are advantages or disadvantages of center tapped / dual winding transformer? I think that the center tapped is cheaper, so I'm leaning towards this option, but if it's a bad idea please prove me wrong.

A whole lot of great advice and kits here; http://sound.westhost.com/projects.htm

yeah, I've been there before, great website!

Some where there is also a LM3886 kit as well ,,I think,,, his site is so big I can't see it right now but it's there somewhere.

I think it's this one: http://sound.westhost.com/project19.htm I think I'm going to use the schematic and etch my own PCB for this one as the shipping costs are kind of big.

That Hood preamp you linked to is not something I would want to use as it has some potential safety issues.

what are the safety issues? Is it because it's a DIY design that hasn't been widely tested or is there something fundamentally wrong with the design? I have no experience with the valve stuff so I don't see those things.

If you want Valves you are better off to go the full build,, All Valves. :tu:

as I said in the post above I'd like to try the valve pre-amp because I've never owned a tube amp, but if it's not worth the hassle then I'll build a SS one!

As for preamps,, heck pick one lol. Wise to stay away from glass stuff for now.

http://sound.westhost.com/project27.htm on the 100W amp page there is a op-amp based pre-amp design - is it going to work fine with lm3886 power amp? I think so, because the lm3886 shouldn't alter the sound, but then again in the project there is a discrete power amp which may alter the sound and make the whole amp sound good? I don't know

If you want Valves you are better off to go the full build,, All Valves

I thought that the pre-amp was mainly responsible for the tone of the sound coming from the amp, do all-valves builds sound better (or at least different) than valve-chip hybrids?

Wise to stay away from glass stuff for now.

What are the reasons that valve builds are more difficult than SS ones? High voltages everywhere, need for careful biasing or some other reasons?

[J M Fahey]

I was going to suggest the excellent Rod Elliott Op Amp guitar preamp, but you already found it.

It can be built on stripboard/Veroboard or even perfboard , and can (and should)  be built and tested on Protoboard first, you can try it before actually designing a board.
Try that with tubes 

Tubes are fine but require lethal voltages , more complex supplies (they also require 6.3V for filaments), sockets which must be mounted one way or another, etc.

None a deal killing problem, of course, but starting full SS is way simpler.

Besides, what Forum are you actually posting in today?   :lmao:

[incurably_optimistic]

Besides, what Forum are you actually posting in today?   :lmao:

:cheesy: hey, just this one, I only posted on diystompboxes before but realised that it probably wasn't a perfect place to ask that question so I came here.

Ok, so i think I have decided to build the mentioned Rod Elliot Op Amp preamp + lm3886 power amp, thanks for the suggestions!

I would like to test it on a breadboard first, but I don't have a +-15V bipolar power supply, so I'd have to build a power supply first, mount the transformer, caps, fuse, rectifier and all that stuff in a chassis and then use this power supply to power a breadboard - this doesn't seem like a great idea (I don't have a bench power supply as this is my first amplifier project and I didn't need it for stompboxes). Unless I can use two small current 12V wall-warts that I have laying around to get a +-12V power supply - will it work? I think it should, because this preamp is essentially one op-amp and one transistor so it's not going to consume too much power? Please correct me if I'm wrong.

I'll probably post some photos and some more questions as I progress with the build if it's ok to do such things here, thanks for your help!

[J M Fahey]

Yes, +/-12V DC is perfectly fine.

In fact, if unregulated , you will have more than 12V, since preamp consmption is VERY low, about 5 mA .

In an emergency, it can run from 2 9V batteries in series, with center tap grounded , effectively +/- 9V rails.

Not as crazy as it looks, you might build such preamp in a box and carry it in your backpack everywhere, either to get into *any*  amp present using your own "known" amp or plain go into a PA Mixer (auxiliary/line input)  and have a "Guitar" sound and EQ, even a little crunch, instead of the boring "straight into the desk" sound.

In fact there are a couple "amp in a box" pedals out there specifically designed for such use, such as Marshall DRP1 , Sansamp Classic, Mesa Boogie V Twin (although the last one is tubed) .

[incurably_optimistic]

allright, so i just breadboarded the preamp and connected it's output into my stereo amplifier - sounds really good and clean and also overdrives nicely with the volume knob cranked up to 9-10! I used two wall-warts, one of them was regulated so I got -12V from it and another was unregulated,  so overall i have +16/-12V - not bad after all. Here are the pictures:

I have to admit I thought about having two separate boxes - one for the preamp, and one for power amp, but I've decided to put them in one box for several reasons - I'm afraid I don't have a box big enough to fit a +-12V transformer in it so I'd have buy a box. Also the cost increases because I would need two power supplys instead of one. The chassis I have is big enough for a power-amp and a preamp, so I find it the most reasonable solution to put them both together in one box. I think I'll add an additional output after a preamp if I need to connect it to someone else's amp.

In other words - yay, it works! Tomorrow I'll go buy myself power supply parts and maybe design a PCB for the preamp - any precautions like "keep the xxx part away from the yyy part" or "make this trace really wide" I should take while doing so? I know that there are some rules about how to design the power amp and power supply boards, but I don't know about anything I should keep in my mind regarding the preamp.

[J M Fahey]

Double congratulations: one for taking the bull by the horns and doing it at once, the other because your Proto preamp looks real cool.  :dbtu:

Now I take one away: I cried when I saw pots and jacks plugged straight into delicate Proto holes 

Ok, now it's done.

I suggest you get a red Sharpie or something and mark the .... ahem!!! .... "expanded" holes ... no transistor/IC/ceramic cap leg will reliably make contact there.

No big deal, just mark them and in future builds always use the same.

And congratulations again. :cheesy:

You can easily get +/- 15V from LM3886 rails with a couple resistors, capacitors and Zeners.

Will you build a head or a combo?

[incurably_optimistic]

Thank you! I have to admit I fried one output transistor when I accidentally attached its emitter directly to the -12V rail instead of 2,2k resistor but shhh!

Whooops, I really didn't think about the implications of plugging the pots and other stuff directly into the protoboard! No wonder I broke so many of those (on the other hand I got zillions of them at the discount at the local store where they cost me about 1$ each but yeah, I see I shouldn't have done that). Now I think that for the future projects I'll just use this breadbord on the front of the first photo exclusively for mounting the offboard elements like pots, jacks (as I like having them firmly attached to something instead of hanging on the wires) and i think the other one will be fine, as there's only one pot in there. But yeah, kind of stupid of me (in my defence I had never done that until I saw someone on the internet mount the pots like that and I thought "what a great idea!"  :lmao:).

You're right, Rod Elliot posted a power supply schematic on his webpage on the 100W amp that does exactly what you said, gets 15V from the rails of the power amp with Zeners, resistors and capacitors:

It was designed for the discrete power amp, but it's gonna work as well with the lm3886 I guess.

I haven't yet decided on whether to make a head or a combo. If I make a head then I'll have to make a separate cabinet as well, because I don't have one, I own a combo amp only - and I wouldn't want to buy a cab, because what is the point of making a DIY amp head if you don't have a DIY amp cabinet?
I'm not planning to gig with this amp, so I think combo would make a more practical and easier/cheaper to make amp. So you made me think about this subject and I think I've finally made a decision - I'll go with combo.

On another note - does it make any sense to use metal film resistors in the build or is it an overkill? Precisely my question is: is there audible difference in noise levels between carbon and metal film resistors in a diy solid state amp? Or maybe I should use them in audio path only?
I've never really heard any big difference in noise levels when using metal film resistors in pedals, but then I may be just deaf and besides, the amps are really a different beast from stompboxes.

[phatt]

Regards Breadboards and pots; You just need a panel to mount the pots and jump wires back to board.
Here is some of my quick and simple test boards for example.

If you look close some pots that have correct pin spacing are mounted directly onto the BB.
Most of those bits come from me scrounging electronic junk like old TV's and VCR gear,, but a lot of that is now drying up.

The switch on the left hand side is just a simple way to implement a true bypass for A/B testing.
I have used this setup for years now it never really got finished as it's always being altered in some way to accommodate a new brain wave. 

Spend a long time on the testing as once you commit to a build it's a lot harder to modify and tweak.

Re metal Resistors,, yes in some cases it can help,, like where you have heaps of gain,,, hi z sections of a circuit yes use metal films.
But keep in mind, even with the best opamps and other pretty colored fancy parts,,, bad layout of PCB can reduce any benefit.
Phil.

[incurably_optimistic]

that's a good idea to have a panel just for breadboarding and testing, I'll make sure to get one in the future!
I'm kind of jealous of the setup you have there (the bench power supply and thise panels), the voltages of the supply are adjustable right? Guess I'll have to build myself one of those bench supplies.
BTW - what exactly is this prototype?

I spent last night on experimenting with different cap/resistor values and opamps and I found one setting that I like the most - I increased the amount of bass by a little and increased the gain of second op-amp stage, also decided to use 5532 instead of tl072.

Once I figured out the sound I like I opened EAGLE and started designing a PCB for the preamp. I finished and etched it today and here's the final effect:


then I decided to wire it up to see if it works:


and put it in the chassis for my testing convenience (don't worry, it's just a temporary wiring):


Surprisingly, it worked from the first try! So I guess I can say that the preamp part of the job is done.

Before I go with the power amp, I want to try to get the reverb tank working, so tomorrow I'll breadboard myself one of those drivers http://sound.westhost.com/project34.htm and see how they work.
But if I get them working, where should I put them in the amp topology? Before or after the preamp? My guess would be before the preamp, is that correct?

[Enzo]

No, make the tone first, then reverberate the sound.  The difference is like echos that are distorted as opposed to distorted sounds that echo.

Look at solid state amps in general, see how they are put together.   Start simple, compare a Peavey Blazer and a Peavey Rage.  Same exact amp, but the Blazer has a reverb added.  They use the same board, one leaves off the reverb parts.  You can look at how the reverb was added in on the schematic.  Same schematic for both units.

http://bmamps.com/Schematics/Peavey/Peavey_Blazer,_Rage_(4-95)_Schematic.pdf


By the way, I have some bench supplies, and am in the process of selling off my shop.  For small circuit development, I have a Heath that would be ideal.  Dual 0-20vDC plus a 5v DC.  Current and voltage meteres and adjustable current limits.  And tracking.  If you have any interest, send me a PM.

[J M Fahey]

Congratulations again on actually protoboarding it, designing the PCB, etching it and building the whole project. :dbtu: :dbtu: :dbtu:

That it actually worked on first try is just the icing on the cake

Lots of people on Forums ask and get answers, but then don't actually go all the way to the end.  :tu:

[phatt]

Agree with JMF,, you are obviously keen,, I can tell by all the holes in the board you have already built a lot of PCB,s  :dbtu:

Regard my circuits on those BB's the bottom one is nothing,, just to show the idea But the top one (with the battery) is one of my early attempts at a decent tone control.
I've built a lot of stuff and most of it would never win awards but that one is one of my few success ideas that actually worked way beyond my expectations.
Just search for PhAbbTone on here as I've posted that schematic a few times the latest version is PhAbbToneV4. The circuit on the BB is the earlier circuit,, the later V4 has better sig/noise performance. It's dead easy to build and can be run as a pedal (battery) circuit or run from a 12-0-12 VDC Supply. Feel free to test it out. :tu:
Phil.

[incurably_optimistic]

Thanks for all the replies and nice words!

That makes sense Enzo, I've never had a reverb pedal of any sort, but that's the same way I used the delay pedals - I put the distortion before them in the pedal chain (who doesn't like that "fading distortion" sound when you switch off the dist. with the delay still on?) and it sounded better that way. Thanks for the link to that Peavey schematic, I'll take a closer look at how it's done in the commercial amps.

Regarding that bench supply - that sounds like the exact thing I need (or I'm going to need in the near future), unfortunately I'm out of money at the moment, so I'll probably write you a PM next month if you have any left!

I've just searched the PhAbbTone and I realised that I've never owned a EQ pedal so i think I'll give it a try after I'm done with this amp! Is the performance of the circuit better with the bipolar supply than with a single 9V supply (more headroom or something)? If so, does it make sense to use some kind of charge pump (maybe icl7660 or something) to get a 9-0-9 from a single 9v supply?

So I'm just in a process of protoboarding the drive and recovery circuit for my reverb tank (figures 2 and 3 from this link http://sound.westhost.com/project34.htm ) but I've just realised that it may not work that great for me, because the tank I have is apparently a high impendance one - I measured the DC resistance of the tank and it was around 60ohm, which would suggest that this is a 600ohm input impendance tank.

Apparently to drive such a tank you need a discrete circuit and something close to lm3886 rails voltage - if the one I'm breadboarding right now doesn't work well, i'm afraid I'll have to wait with the reverb part until I'm done with the power supply, which is less than optimal for me.

EDIT I forgot that I have some small transformers that I salvaged from broken laptop power supplies and wall warts - I'll try using them if the circuit doesn't drive the spring enough!